The long-term goal of the proposed research is to gain a better understanding of the mechanism of neurotransmitter release and of the factors that mediate changes in transmitter release with repetitive stimulation. Experiments will be done using the frog (Rana pipiens) sartorius nerve-muscle preparation, and in some cases the rat phrenic nerve-medidiaphragm preparation. Standard intracellular and extracellular recording techniques will be used to examine the effect of repetitive stimulation on end-plate potential amplitude and miniature end-plate potential frequency. The first objective of the proposed research will be to characterize the factors or processes that act to increase transmitter release at the frog neuromuscular junction under conditions of reduced quantal content (decreased Ca, increased Mg). Two hypotheses that have been proposed to account for stimulation-induced increases in transmitter release will be examined: (1) the Ca accumulation hypothesis, which proposes that the increase in release that occurs during and following repetitive stimulation arises from an accumulation of Ca++ in he nerve terminal; and (2) the Na accumulation hypothesis, which proposes that this increase in release arises from an accumulation of Na in the nerve terminal. The second objective will be to characterize depression of transmitter release at the frog neuromuscular junction. These experiments, which will be done under conditions of normal or increased quantal content, will be aimed at determining the kinetic properties of depression and the effect of depression on miniature end-plate potential frequency. The third objective will be to examine the role of presynaptic muscarinic and catecholamine receptors in regulating transmitter release at the frog and rat neuromuscular junctions. These experiments will involve looking at the effects of muscarinic and catecholamine agonists and antagonists on release and on the factors that act to increase release during and following repetitive stimulation. It is hoped that the knowledge gained from this study will contribute to a better understanding of the processes involved in the functional contact between excitable cells and of the role these processes play in information processing in the nervous system.